User conflicts and resistance behaviours represent one of the major managerial difficulties during Information Systems (IS) implementation. Although numerous IS studies tackle user resistance and conflicts by anticipating, minimising or even amplifying these negative behaviours, little research has taken into consideration the contagion mechanisms of conflicts and examined why and how users similarly resist to entirely different Information Technology (IT) projects. The IS literature is sparse on addressing the impact of resistance behaviours that occurred in the past on the new IT. IS managers need to anticipate potential causes for project failure because of ongoing resistance behaviours toward the new IT, but also because of resistance behaviours occurred in the past and persist until today, or are likely to reappear. For IS researchers and practitioners, my research gives greater attention to issues related to current and prior conflicts occurring in a firm?s IT project portfolio. Therefore, we particularly mobilise the theories of contagion and Inoculation which have been so far weakly used in the field of information systems. The empirical part consists of a three-year action research project conducted at Efficient Innovation Corporation (a French management consulting firm). Firstly, my observations reveal that employees expressing task-oriented conflicts towards an IT deployment project were likely to use a bypassing strategy, hiding socio-political conflicts. Secondly, we were able to identify a phenomenon of conflict contagion between two IT projects despite their strong differences in terms of objectives, functional perimeters and users. Thirdly, the "Inoculation" techniques that we implemented succeeded to stall and limit the diffusion of resistance behaviours toward IT. The underlying message of this thesis is to consider prior resistance behaviours and conflict contagion as key processes integrated into IT implementation. Accordingly, we introduce the concept of "Resistance Path Dependency", and propose a conceptual model of possible conflict contagion mechanisms between different teams and IT projects.